1. Field of the Invention
The present invention relates to a high efficiency, high energy slurry mixer used primarily to mix oil field cement in a recirculating system for cementing the casing in oil and gas wells. Specifically, the present invention employs continuous recirculation via a central recirculation line and via annular recirculation jets that are spaced alternately with annular adjustable mix water jets.
2. Description of the Related Art
Utilization of cement within oil wells, particularly in the cementing of casing therein, has been under development since the early 1900""s. Two of the purposes of placing cement into the annular space between the casing and the formation are to support the casing within the well, and to seal off undesirable formation fluids.
Casing is typically secured in the well bore by the cement which is mixed at the surface, then pumped down the open center of the casing string and thence back up the annular space which exists between the outer diameter of the casing and the inner diameter of the oil well bore. A displacement fluid, such as drilling mud, is pumped behind the cement to push the cement to the desired location. In many oil and gas well applications it is often necessary to provide cement mixers which will rapidly prepare large quantities of material to be pumped into the well by a batch or continuous process until a sufficient predetermined quantity has been applied.
In either case, the process usually begins with the material being pre-prepared by dry blending and then adding water at the well site. Batch mixing is one form of system to obtain a satisfactory slurry, but batch mixing requires an initial outlay of a large amount of equipment, people, and space. In offshore operations, space and weight capacity are expensive. Batch mixers use valuable space and add to rig weight. Typically, large tanks with rotary paddle type mixers, although being able to adequately perform the mixing operations, have not been efficient in term of space, numbers of people required or equipment costs where large volumes of mixing must be done at the well site.
The quality of the cement slurry placement process involves the completeness of the mixing process and the pumping rate which can affect the bond between the casing and the well bore. The completeness of the mixing process depends on the efficiency of wetting all the dry bulk particles. The pumping rate affects the bond by having an important effect on mud displacement efficiency. There are many other factors that affect the quality of the cementing process.
Many types of cement mixers have been known in the prior art. For example, jet-type mixers and vortex mixers such as those disclosed in U.S. Pat. Nos. 3,201,093 and 3,741,533 have been used with considerable success but have not necessarily been successful in continuously mixing cement slurries. Such jet or eductor type mixers worked reasonably well when slurry designs were simple. With the more enhanced slurry designs of today, the jet mixer cannot adequately mix these slurries. Early type mixers generally had centrally located water jets, while later models added a recirculation flow in combination with the central water jets.
Continuous recirculation mixers were developed to overcome some of the deficiencies of the jet type and batch mixers. These systems mix dry cement and water in an inlet mixer, the output going to a tank for agitation with excess slurry flowing over a weir to an averaging tank, which may be agitated, thence pumped into the well. Typically, a portion of the mixed slurry was recirculated from the mixing tank and directed back into a modified jet mixer. Thus, newly delivered dry bulk cement was wetted both by water and recirculated cement. This provided additional mixing energy that enabled satisfactory mixing. These type mixers were first introduced during the early 1970""s. Since that time, cement slurry design has evolved into the use of more complex slurries that earlier continuous mixing systems are unable to mix satisfactorily. Thixotropic slurries with very low xe2x80x9cfree waterxe2x80x9d requirements have evolved for the deep, high temperature, high pressure gas wells. It seems as though the industry is constantly testing the ability of mixers by developing even more difficult to mix slurries.
Although prior inventions have taught use of centrally located recirculation jets, or alternately, annularly located recirculation jets, none of the prior art teaches or suggests the desirability of providing both centrally located and annularly located recirculation jets. The present invention incorporates this arrangement and adds mix water jets located between the discrete annularly located recirculation jets so that flows from the annular recirculation jet and the water jets overlap each other. With this arrangement, recirculation rate in the present mixer is independent of slurry design or mixing rate, but is only dependent upon recirculating pump capacity and mixer design.
One of applicant""s previous inventions, as taught in U.S. Pat. No. 5,046,855, provided a combination of annular water jets and recirculation jets with no centrally located jets. Another of applicant""s previous inventions, as taught in U.S. Pat. No. 5,571,281, included annular recirculation jets and a centrally located water jet. The short coming of the U.S. Pat. No. 5,571,281 mixer was when mixing slurries that had a low water requirement, i.e. small number of gallons of water per sack of cement, or when the mixer was used in a batch mode, there is insufficient energy to effectively wet all the incoming dry bulk cement. The present invention addresses this problem by having both centrally located and annularly located recirculation jets which operate all the time and provide good mixing regardless of slurry design or operation in continuous or batch modes.
Prior art mixers, including both the U.S. Pat. No. 5,046,855 mixer and the U.S. Pat. No. 5,571,281 mixer, utilize discrete annular recirculation jets, i.e. the former having two and the latter having four. The use of discrete jets is practical, but allows the potential bulk cement to by pass the jets, and thus discharge without becoming wet. The present invention addresses this problem by having the mix water jets located between the discrete recirculation jets. These mix water jets overlap the discrete recirculation jets, thus providing 100% coverage of the flow path of the dry bulk cement.
Also, the U.S. Pat. No. 5,046,855 mixer also suffered from discrete mix water jets. That design included six sets of three jets each, for a total of eighteen jets. These jets opened consecutively as increasing water rate was required. Coverage was good when all jets were open, but when only the first set of jets was operational due to low mix rate or low water requirement or both, coverage was poor and mixing quality suffered.
The U.S. Pat. No. 5,571,281 invention provided a continuous circumferential and diverging flow pattern for mix water which worked well when relatively high water rates were required but provided little mixing energy when low water requirements existed or when batch mixing. The present invention provides good coverage and mixing energy regardless of mix water requirement or while batch mixing.
The prior U.S. Pat. No. 5,571,281 mixer provides xe2x80x9cbaffledxe2x80x9d annular space through which recirculated flow passes. This design is unnecessarily restrictive to the recirculation flow and has a low coefficient of discharge. The flow path of the present invention for annular recirculated flow is more streamlined and thus has a higher coefficient of discharge. With a higher coefficient of discharge, this means that pressure head is converted to velocity more efficiently and therefore provides the same mixing energy with less input horsepower.
One object of the present invention is to improve the mixing capabilities as compared to prior art mixers. The present invention will provide more effective and efficient mixing over a wide range of conditions, including both batch mixing and continuous mixing modes.
A second object of the present invention is to have effective mixing while only recirculating water and/or slurry from a mix tank and not adding additional water. Present technology performance during this mode of operation is significantly degraded. This operation is typical while starting the recirculating process or while batch mixing.
A third object of the present invention is to provide both a centrally located recirculation jet and a plurality of equally spaced annular recirculation jets. Bulk cement enters the mixer and encounters high energy jets from the center and from annular jets. These jets have trajectory angles which intercept the dry bulk, breaking it apart and effectively wetting the incoming dry bulk.
A fourth object of the present invention is to provide a plurality of annular and adjustable water jets located at alternate positions from the annular recirculation jets. These jets, in combination with the recirculation annular jets, provide improved mixing and more effective wetting of the bulk cement.
A fifth object of the present invention is to provide a mixing system that provides more predictable slurry properties due to improved and effective mixing.
A sixth object of the present invention is to provide a mixer which provides high mixing energy while consuming less energy.
A seventh object of the present invention is to allow the use of more than one recirculation pump source, further optimizing the use of mixing energy sources.
These and other objects will become more apparent upon further review of referenced drawings, detailed description, and claims submitted herewith.
The present invention is a cement mixing system and mixer for mixing cement that will be used in cementing oil wells. A recirculation pump recirculates the contents of a cement mixing tank to the mixer via annular recirculation flow inlets provided on the mixer, and also via a central recirculation inlet provided on the mixer.
The mixer is provided with the bulk cement inlet, the central recirculation inlet and associated central recirculation line, the mix water inlet, the annular recirculation flow inlets, a mix water adjustment input means, and the slurry outlet.
The centrally located central recirculation line discharges through a nozzle into a mixing chamber provided within the mixer.
The annular recirculation flow inlets connect to a recirculation manifold chamber which is defined by the inside diameter of an outer housing of the mixer and the outside diameter of the fixed part of the water metering means. The water metering means consists of the fixed part that cooperates with a movable part. The movable part is comprised of a rotatable water metering valve element and its attached mix water adjustment input means. The recirculation manifold chamber is connected to parallel multiple recirculation outlets where each recirculation outlet is defined by two surfaces within the mixer. The recirculation outlets discharge into the mixing chamber.
The mix water inlet is connected to a mix water manifold chamber which is defined by the I.D. of the fixed part of the water metering means and the O.D. of the rotatable water metering valve element. The mix water manifold chamber is connected to parallel multiple and elongated jet outlets. Each elongated jet outlet is formed by a matching set of elongated jet openings, with one of the elongated jet openings of each set provided in the rotatable water metering valve element and with a cooperating and associated elongated jet opening provided in the fixed part of the water metering means. Each matching set of elongated jet outlets are located in such a way that if the rotatable water metering valve element is rotated, the size of orifice of each of the elongated jet outlets is changed. Mix water flows out of the mix water manifold chamber via the adjustable jet outlets which discharge into the mixing chamber. The jet outlets which discharge mix water into the mix chamber are located so that they alternate with and are evenly spaced relative to the annular flow recirculation outlets. The evenly spaced and alternating jet outlets deliver mix water annularly to the mixing chamber and recirculation outlets also deliver recirculation flow annularly to the mixing chamber. The discharge nozzle of the central recirculation line delivers recirculation flow centrally within the mixing chamber.
The elongated metering slots of the rotatable water metering valve element are equally spaced. The element is provided with threaded holes as means to attach a mix water adjustment input means to the rotatable water metering valve element via threaded fasteners to rotate the rotatable water metering valve element in order to adjust the flow of mix water passing through the elongated jet outlets. Grooves are provided in the rotatable water metering valve element to accommodate pressure seals to contain water pressure within the mix water manifold chamber.
Elongated metering slots in the fixed part of the water metering means are equally spaced and alternately located between the recirculation outlets. The water discharge chamber is connected to the metering slots. Each of the recirculation outlets changes shape and decreases in cross sectional area as it approaches the mixing chamber, thereby increasing fluid velocity as the recirculated slurry approaches the mixing chamber. A groove is provided for a seal to prevent mix water from entering the I.D. of the chamber
The centrally located central recirculation line conveys recirculation flow to a discharge nozzle provided in the line. Two inlet elbows that attach to the central recirculation line are arranged at 90 degree angles to each other so as to cause the flow of recirculation within the line to rotate. Therefore when discharged from the nozzle, the recirculation fluid continues to rotate. The rotational flow tends to diverge as it discharges from the nozzle in a pattern that enhances mixing.